Conventionally are already known a projection-type image display apparatus, which mounts an optical unit for modulating an intensity of light emitted from a light source through an image display element(s); i.e., the gradation is changed depending upon an image signal, thereby forming an optical image, and projecting that optical image, enlargedly, by means of a projection lens. And, as such the image display element, there are already known a transmitting-type liquid crystal panel, a reflection-type liquid crystal panel, and a micro mirror panel, and so on.
Among of those mentioned above, within the optical units of using the reflection-type liquid crystal panel(s), there is applied a polarization beam splitter (hereinafter, being called as “PBS”) prism, functioning as a polarizer and an analyzer in common, as is described in FIG. 12 of Patent Document 1 (Japanese Patent Laying-Open No. 2001-142028 (2001), for example. Within this, the light emitted from the light source is uniformed or aligned into a predetermined direction in polarization direction, by means of a polarization light conversion element, to be incident upon the PBS prism. The incident light is reflected upon a PBS film surface thereof, to be incident upon the reflection-type liquid crystal panel. The light incident upon the reflection-type liquid crystal panel is modulated in the polarization condition for each pixel depending upon the image signal. The light reflected upon the reflection-type liquid crystal panel is incident upon the PBS prism, again, so that only the light modulated in the polarization condition can pass through; thereby being projected, enlargedly, by means of that projection lens.
However, with this technology, a ¼ wavelength plate is necessary for lessoning a leakage light, which is caused when an oblique or inclined light is incident upon the PBS prism, not being parallel with a surface (a main incident surface) thereof, which is defined by an optical axis and the PBS film surface. However, because of an incompleteness of the effect thereof, there is still a drawback that contrast cannot be enhanced.
Then, in Non-Patent Document 1, i.e., “Optical Flat Polarizing Beamsplitters”: Moxtech Co., U.S.A., a catalogue No. PBF02A (May 2002), there is introduced an example of applying the reflection-type polarization plate, as the polarizer and the analyzer, in common, reflecting a light polarized in parallel to the direction of grating, while transmitting a light polarized in orthogonal to that direction of grating, due to diffraction of the optical grating thereof.